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Room Acoustics

Reverberation Time

The most important acoustic characteristic of a closed room is its reverberation time. This is the amount of time which elapses until the sound pressure level has fallen by 60 dB and is dependent on the absorption qualities of the walls, floor, ceiling and furnishings as well as the volume of the room in question. The reverberation time also varies with frequency, as materials, such as stone, wood, carpets or other textiles absorb frequencies in different amounts.

For balanced, lively and not too dry room acoustics in the living room, a linear reverberation time across all frequencies of approx. 0.3s - 0.4s (according to the size of the room) should be targeted. If the reverberation time for low frequencies is much longer, then the room resonances are only slightly dampened and the bass response rumbles and lacks precision – it could even be boomy. If the reverberation times for the mid and high frequencies are also much longer, then the whole sound field loses transparency and the positions of the individual instruments become vague.

As a rule the reverberation times for low frequencies are much too long, the mid range times a little too long and the high frequencies are a little short. This effect is more pronounced the less absorbing textiles there are in the room, especially in modern rooms furnished sparingly with light textiles, perhaps without curtains and with parquet or ceramic flooring. In rooms with heavy curtains, carpets and a lot of soft furnishings this is less of a problem.

Room Resonance

Sound transmission at low frequencies is governed by the resonance of the volume of air a room contains. The frequencies at which these resonances occur and the distribution of their sound pressure levels is dependent upon the geometry and size of the room in question. Attaining a high quality of sound reproduction is a question of dampening the room resonances sufficiently, otherwise some of the notes the bass instruments produce will seem unprecise and too loud. Optimal loudspeaker positioning can help to reduce this effect, but the easiest way is to dampen the room resonance modes using low frequency absorbers.

Reflections, Echos

A matter of great importance when reproducing music through loudspeakers is the relationship of direct sound to that reflected to the listener via walls etc., because these parts of the sound field can be distorted due to phase-shift interference patterns. These effects become more noticeable the stronger the reflections are, and the sooner they reach the listener, which is why the early reflections from the walls at the sides of the loudspeaker cabinets are most critical. These can, however, be easily avoided by deploying mid/high absorbers.

Echos are a seldom occurrence in living rooms. Reflections are audible as echos only when they are strong enough, and arrive at least 50 ms later than the direct sound waves. This time difference corresponds to a distance of 17 metres, which means that the sound is reflected back and forth between the walls. The most problems are caused by hard, flat and smooth walls situated parallel and opposite each other. This problem can also be solved by the positioning of mid/high absorbers.